1. Field of the Invention
The present invention relates to a gas sensor such as a catalytic combustion hydrogen sensor which is provided in a fuel-cell vehicle.
Priority is claimed on Japanese Patent Application No. 2005-107806, filed Apr. 4, 2005 and Japanese Patent Application No. 2005-176228, filed Jun. 16, 2005, the contents of which are incorporated herein by reference.
2. Description of Related Art
Conventionally, a known solid polymer membrane fuel cell has a stack including a plurality of cells which are constituted by a solid polymer electrolyte membrane being sandwiched by a fuel electrode and an oxygen electrode (below, simply referred to as the “fuel cell”). Hydrogen is supplied to the fuel electrode as fuel and oxygen is supplied to the oxygen electrode as oxidizing gas. Hydrogen ions generated by a catalytic reaction at the fuel electrode move to the oxygen electrode through the solid polymer electrolyte membrane. Electricity is generated by an electrochemical reaction of the hydrogen ions and oxygen at the oxygen electrode.
In fuel cells including the aforementioned one, a protection device having a hydrogen detector (a gas sensor) provided in an exhaust system of the oxygen electrode of the fuel cell, wherein the protection device stops fuel supply when the gas sensor detects that hydrogen of the fuel electrode has leaked to the oxygen electrode through the solid polymer electrolyte membrane is conventionally known (see, for example, Japanese Unexamined Patent Application, First Publication No. H06-223850).
As the hydrogen detector, a catalytic combustion hydrogen detector having a gas detection element made of a catalytic substance such as platinum and a temperature compensation element is known. When hydrogen contacts the catalytic substance such as platinum, it combusts and heat is generated. The gas detection element reaches high temperature due to the heat and the temperature compensation element, which is at under ambient temperature, is caused to be at relatively low temperature. This hydrogen detector measures a concentration of hydrogen gas in accordance with a difference in electric resistance between the gas detection element and the temperature compensation element generated by the aforementioned process.
In addition, a gas sensor having a tube-shaped case which houses the gas detection element and the temperature compensation element and a tube-shaped heater provided in the case is also known. The inner surface of the heater releases heat and surrounds the gas detection element and the temperature compensation element (PCT International Publication No. WO2003/042678).
Incidentally, in the aforementioned fuel cells, offgas of the fuel cell, especially from the oxygen electrode reaches high humidity because water (hereinbelow, simply referred to as “humidification water”) is mixed into the reaction gas (such as hydrogen or oxygen) which is supplied to the fuel cell by a humidification device or the like, in order to maintain the ion-conductivity of the solid polymer electrolyte membrane. Furthermore, water is produced by an electrochemical reaction (so-called “produced water”) when the fuel cell operates.
Therefore, in the protection devices disclosed in the aforementioned publications, condensation may occur on the hydrogen detection element provided in a flow path of the offgas from the fuel cell, which has high humidity. In this case, the hydrogen detection element may deteriorate or be damaged. Especially, the water in the offgas readily condenses in the aforementioned solid polymer membrane fuel cell because the fuel cell usually operates at a temperature lower than the one where water becomes vapor and offgas having high humidity is exhausted from the fuel cell. Therefore, when the catalytic combustion hydrogen detector is provided in the exhaust system of the oxygen electrode of the fuel cell, the gas detection element may be broken or decline in sensitivity in a case where topically inhomogeneous temperature of the surface of the gas detection element caused when current is applied to the gas detection element with the humidification water, produced water or the like being deposited on the surface thereof.
When a heater provided for lowering a humidity of the case is provided in the case which houses the gas detection element and the temperature compensation element to solve the aforementioned problem as the above publication discloses, the number of parts of the gas sensor increases and the gas sensor must be large to ensure a detection chamber of a desired size. Furthermore, a notch is made on the wall of the tube-shaped heater along the axial direction to connect two leads for applying current. Therefore, a traverse sectional shape of the heater becomes almost a C-shape and a temperature near the notch becomes relatively low. This may cause inhomogeneous temperature of the surroundings of the gas detection element and the temperature compensation element and may lead to condensation.